Fuels other than gasoline and diesel are increasingly being used as an energy source for internal combustion engines because of possible reduction in certain regulated emissions. In particular, methanol and gasoline blends are commonly encountered. The engine air/fuel ratio control system must be able to detect these blends and make adjustments to the engine control parameters in order to maintain optimum operation.
In particular, measuring the fuel blending ratio is critical because of the relationship between the blending ratio and the stoichiometric air/fuel ratio. As the blending ratio changes, and thus the chemical makeup of the fuel changes, the stoichiometric air/fuel ratio changes. Unless the air/fuel control system has information regarding the fuel blend, the control system must rely on exhaust gas sensor feedback signals to maintain stoichiometric combustion. While this approach will maintain stoichiometric combustion, significant errors can be encountered during transient operation.
When using a fuel blend sensor to eliminate the transient problems related to the feedback control, another error can be experienced related to the location of the sensor. Typically, the sensor is placed upstream of the fuel rail and fuel injectors, which deliver fuel to the engine. When an unknown blend of new fuel is added to the fuel tank, it takes a certain interval for the new fuel blend to reach the sensor. Then, it takes another interval before the new fuel blend reaches the fuel injectors. The engine control system must be able to compensate for this delay so that the proper air/fuel ratio is maintained when the new fuel blend finally reaches the fuel injectors.
When the fuel system comprises a conventional mechanical return fuel system, a substantially constant flow rate of fuel is continually recirculated regardless of engine operating conditions. Thus, it takes a predetermined time for the new fuel to move from the sensor to the fuel rail. Therefore, the engine control system can predict when the new fuel blend will reach the engine and compensate accordingly. Such a system is disclosed in the U.S. Pat. No. 5,325,836.
The inventors herein have recognized numerous disadvantages with the above approaches. One disadvantage is that when the engine comprises an electronic returnless fuel system, the time for fuel to move from the sensor to the fuel rail is not constant, but rather a function of engine operating conditions. Another disadvantage is that, because the time delay is a function of engine operating conditions, the above described method is only valid for constant engine operating parameters. Yet another disadvantage is that, because the fuel blend does not change instantaneously, but gradually as the new fuel and old fuel mix, time based compensations are insufficient.